adb: mark kMaxProcessNameLength as constexpr.

Otherwise, the build fails with -O0.

Test: mma with -O0 in adb_defaults
Change-Id: Id10e320afc183eda5b46555b3b50dd8ffd84a700
diff --git a/adb/libs/adbconnection/include/adbconnection/process_info.h b/adb/libs/adbconnection/include/adbconnection/process_info.h
index 86d3259..d226699 100644
--- a/adb/libs/adbconnection/include/adbconnection/process_info.h
+++ b/adb/libs/adbconnection/include/adbconnection/process_info.h
@@ -21,7 +21,7 @@
 #include <string>
 
 struct ProcessInfo {
-  const static size_t kMaxArchNameLength = 16;
+  static constexpr size_t kMaxArchNameLength = 16;
 
   uint64_t pid;
   bool debuggable;